This invention relates to cable television ("CATV") systems, and more particularly to apparatus for verifying that data transmitted in a CATV system for such purposes as system control is correctly transmitted and received.
In a conventional CATV system, a head end transmits CATV signals along a trunk cable. Connected to the trunk cable are feeder cables, to which in turn are connected drop cables. The drop cables connect the feeder lines to converter/tuner units located in subscribers' houses. When a subscriber desires to tune to and view a particular TV channel, he or she keys the requested channel into the converter/tuner. If the subscriber has subscribed with the CATV service company for the requested channel, the converter/tuner tunes the selected channel from among a plurality of channels available on the drop cable and converts the selected channel to a form and frequency suitable for viewing on the subscriber's television set. If the subscriber has not subscribed for the requested channel, the converter/tuner will not tune to or enable the viewing of the requested channel.
At least two difficulties arise with a conventional CATV system as described above. First, the subscriber may modify the converter/tuner to allow the subscriber to view CATV channels for which the subscriber has not subscribed, thus resulting in a loss of revenue to the CATV service company. Second, the subscriber may remove the expensive converter/tuner. Again, the CATV service company incurs a loss.
To solve these problems of modification and removal of the converter/tuner, it is possible to design a CATV system in which the expensive converter/tuner is removed from the subscriber's house and placed in an external control unit mounted, e.g., on or adjacent to a nearby telephone or utility pole. Each external control unit may include a plurality of converter/tuners, each connected to a respective one of a plurality of subscribers' houses by a respective one of a plurality of drop cables. In each subscriber's house is a subscriber processing unit which connects to the subscriber's drop cable and to the subscriber's television set. The subscriber may key into the subscriber processing unit a TV channel request. Thereupon, the subscriber processing unit transmits the request to the external control unit via the drop cable in the form of a modulated data signal. If the subscriber is authorized to view the requested channel (i.e., if the subscriber has subscribed and paid for the channel), the converter/tuner located in the external control unit which is associated with the subscriber tunes to the requested channel and transmits the channel to the subscriber via the drop cable.
Although the foregoing CATV system solves the problems of modification and removal of converter/tuners, problems may occur in communicating channel requests from the subscriber processing unit to the external control unit. For example, noise or poor connections along the communication path between the external control unit and the subscriber processing unit may degrade or modify the data signal containing the subscriber's TV channel request. As a result, a different channel request from the actual channel request might be received by the external control unit, or no channel request might be received. In either case, the subscriber does not receive the TV channel which he keyed into his subscriber processing unit.